Device for bending a plasma flame

ABSTRACT

A PLASMA FLAME GENERATOR IS USED AS AN EXCITATION SOURCE FOR SPECTROSCOPIC ANALYSIS, THAT IS, A PLASMA FLAME GENERATOR HAVING A DEVICE FOR BENDING A PLASMA FLAME, IN WHICH A PAIR OF AUXILIARY ELECTRODES ARE DISPOSED SO AS TO ENCIRCLE THE PLASMA FLAME ALONG THE CENTRAL AXIS OF THE PLASMA FLAME AND A DC VOLTAGE IS APPLIED TO SAID AUXILIARY ELECTRODES FOR UTILIZING THE DRIFT OF IONS ALONG THE ELECTRIC LINES OF FORCE TO BEND THE PLASMA FLAME IN A PREDETERMINED DIRECTION.

Inventors Manabu Yamamoto Odawara-shi; Seilchi Murayarna; HlromitsuMatsuno, Hachlojl-shi, Japan Appl. No 791,433

Filed Jan. 15, 1969 Patented June 28, 1971 Assignee Hitachi, Ltd.,Tokyo, Japan Priority Jan. 19, 1968 Japan 43/2629 DEVICE FOR BENDING APLASMA FLAME 10 Claims, 11 Drawing Figs.

U.S.Cl 315/111,

219/121, 356/85 Int. Cl l-l0lj 7/24 Field of Search 219/121 [56]References Cited UNITED STATES PATENTS 2,945,119 7/1960 Blackman 219/1233,277,265 10/1966 Reboux 219/121X 3,353,060 11/1967 Yamamoto et al.315/111 FOREIGN PATENTS 1,345,152 10/1963 France 315/111 PrimaryExaminer-Raymond F. l-lossfeld Att0rneyCraig, Antonelli, Stewart andHill ABSTRACT: A plasma flame generator is used as an excitation sourcefor spectroscopic analysis, that is, a plasma flame generator having adevice for bending a plasma flame, in which a pair of auxiliaryelectrodes are disposed so as to encircle the plasma flame along thecentral axis of the plasma flame and a DC voltage is applied to saidauxiliary electrodes for utilizing the drift of ions along the electriclines of force to bend the plasma flame in a predetermined direction.

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SHEU R [If 4 INVENTORS nu llnumrrau M/Ifsmva BY M a?! ATTORNEYS DEVICEFOR BENDING A PLASMA FLAME BACKGROUND OF THE INVENTION FIELD 'OF THEINVENTION This invention relates to an excitation source forspectroscopic analysis and more particularly to a plasma flame generatorin which auxiliary electrodes are disposed so as to encircle a plasmaflameand a DC voltage is applied to said auxiliary electrodes to makeions in the plasma flame drift in a predetermined direction.

Spectroscopic analysis which makes a quantitative and qualitativeanalysis of a substance by means of emission or absorption spectra ofelements to be analyzed in a substance introduced into a flame hasgenerally a high accuracy and good reproducibility and is widely used.For spectroscopic analysis flames such as the following are used: theplasma flame produced by high frequency torch discharges, theoxyhydrogen flame, combustion flames and DC plasma jets.

BRIEF DESCRIPTION OF THE DRAWING FIG. la is a diagram illustrating theconstruction of a spectroscopic analysis device.

FIG. lb is an enlarged view illustrating the main portion of therelation between a plasma flame and a spectrometer of the spectroscopicanalysis device shown in FIG. la.

FIG. 2 is an intensity of the spectral line vs. wavelengthcharacteristic diagram of an element detected by the detector of thespectroscopic analysis device.

FIG. 3a is a diagram illustrating the observation position of the plasmaflame.

FIG. 3b illustrates the main portion of a conventional high frequencyplasma torch generator.

FIGS. 4a and 4b are diagrams illustrating the plasma flame state whenthe spectroscopic analysis is carried out by a conventional plasma flamegenerator.

FIG. 5 is a cross section of an embodiment of the plasma flame generatorof the present invention.

FIG. 6 is a cross section of another embodiment of the plasma flamegenerator of the present invention.

FIGS. 7a and 7b are horizontal and vertical cross sections respectivelyshowing the main portion of another embodiment of the present invention.

DESCRIPTION OF THE PRIOR ART First of all, the flame must be stabilizedin order to improve the accuracy and detection limit of thespectroscopic analysis. For that purpose, the flame was hitherto jettedupward in the direction of gravity as shown in FIG. lb. Referring toFIG. 1a showing a horizontal cross section of a spectroscopic analysisdevice, the reference numeral I indicates a flame, 2 is a spectromcter,3 and 4 are entrance and exit slits respectively, 5 is a light detectorand 6 is a concave grating. When a spectrum is swept by rotating theconcave grating of such a device, a wavelength vs. output waveform asshown in FIG. 2 appears at the light detector 5. Referring to FIG. 2,the reference mark S indicates the intensity of an emission line from anelement to be analyzed (hereinafter referred to as the spectral-lineintensity), N indicates the continuum emitted from the flame I (ofcourse the band spectrum which cannot be resolved by the spectrometerused is also included), and AN indicates the fluctuation of thecontinuum caused by instability of the flame 1 (hereinafter referred toas noise). Now, the spectral-line intensity 5 decreases with a decreasein concentration of the elements to be analyzed. As described above thespectral-line intensity S is determined by the amount of elements to beanalyzed, and the analysis cannot be carried out when the spectral-lineintensity S becomes substantially equal to the noise AN of thecontinuum. Therefore, S/AN must be made large in order to analyzeextremely small quantities of elements to be analyzed. For that purpose;

I. The flame must be stabilized in order to make AN small.

2. It is necessary to use an appropriate portion of the flame.

The foregoing can be explained by fixing the conditions of thespectrometer and light detector. The spectral-line intensity S of anelement to be analyzed is a function of the temperature of the flame I.The noise AN depends on that portion of the flame l where theobservation is made and is substantially proportional to N when theobservation position is fixed, since the noise AN is caused by thefluctuation of the continuum intensity N due to, for example, flickeringof the flame.

The central portion of the flame of the excitation source forspectroscopic analysis has a high temperature and its periphery has alower temperature since the periphery is cooled. Therefore, S/AN differsdepending upon the direction perpendicular to the central axis 0 of theflame (hereinafter referred to as the lateral observation position) asshown in FIG. 3a, for example, S/AN differs whether the center 0 or theperiphery is observed. Moreover, the lateral observation position whereS/AN becomes maximum differs depending upon the species of the elementsto be analyzed or what form of compound of the elements is included in asample, since parameters of the spectral-line intensity S (thedissociation energy of the sample compound, the excitation energy of thespectral-line, etc.) which are a function of the temperature differdepending upon the elements to be analyzed and the sample compounds, andthe continuum intensity N differs dcpending upon the wavelength of thespectral-line intensity.

And in the case of an analysis using atomic absorption which is a methodof spectroscopic analysis using the phenomenon wherein the spectral lineemitted by an element in a high temperature portion is absorbed by anatom of the same element in a low temperature portion, there is also anoptimum lateral observation position depending upon the kind of elementsto be analyzed or of the compounds since in this case also the densityof the absorbing atoms or others differ depending upon the position inthe flame.

In short, when spectroscopic analysis is carried out, it is veryimportant in order to increase the accuracy and detection limit of thespectroscopic analysis to suitably select the lateral observationposition depending upon the kind of elements to be analyzed and of thecompounds. However, it is very difficult in a conventional device tochange the lateral observation position. That is, the image of the flamemust first be focused on the slit of the spectrometer, then the flamemust be moved in a lateral direction for that purpose, but the wholeflame generator (such as magnetron, waveguides, water pipes and gaspipes in the case of high frequency torch discharges, and such as gaspipes in the case of combustion flames) must be moved in order to movethe plasma flame in the lateral direction, and quick movement isimpossible. Furthermore, it is generally very difficult to make aspectrometer.movable since a spectrometer is generally a large and heavyprecision machine. In particular, as shown in FIG. 3b, a high frequencytorch discharge generator is so constructed that the plasma flame l issurrounded by an outer conductor 9 by extending the outer conductor 9 ofa coaxial waveguide upward beyond the end of an inner conductor 10 inorder to lessen the high frequency electric power loss due to radiationfrom the discharge plasma and to prevent the plasma flame from becomingunstable by the turbulence of the gas stream. Then, observation is madethrough a small observation window 8 provided in the outer conductor 9.Thus it is very difficult to change the lateral observation position ofthe flame.

Thirdly, the resolving power of the spectrometer must be increased bymaking the width of slits 3 and 4 of the spectrometer narrow in order toimprove the detection limit. That is, first the detection limit can beimproved by removing the effect of near lines. Next, S/N and then S/ANcan be made large by making the width of the slits narrow enough sincethe spectralline intensity S is almost linearly proportional to the slitwidth and the continuum intensity N is almost proportional to the squareof the slit width. Thus the detection limit can be improved.

However, when the excitation source flame is not bright enough, andparticularly when the analysis of elements having spectral lines in theshort wavelength region of about 2000 A where the absorption by thefocusing system is large, for example, carbon, phosphorus, sulfur, etc.is carried out, the width of the slits cannot be made narrow enoughsince there is a limit in light detecting capacity of the lightdetecting portion 5. In particular, in the case of the lateralobservation position, the light from the flame cannot be utilizedeffectively since the flame is long in the direction perpendicular tothe optical axis and the light arriving at the spectrometer is weak.

However, if the flame is observed from the direction of the axis of theflame, the quantity of light increases by the ratio between the lengthof the flame in its axial direction and its diameter than when the flameis observed from the lateral direction. This ratio is about in the caseof a usually used plasma flame by high frequency torch discharge orcombustion flames and the light becomes about ten times stronger. Inthis case the width of the slit can be made narrow enough and theaccuracy and detection limit can be improved.

As shown in FIG. 4a, the flame l is jetted perpendicular to thedirection of gravity in order to observe the flame from its axialdirection, but in this case a problem arises, that is, the flame is bentupward by the convection of gas as shown in FIG. 4b to cause instabilityand the detection limit decreases.

As has been described, the observation can be made suitably from theaxial direction of the flame or from the direction perpendicular to theaxis of the flame depending upon the intensity of the light, but ineither case problems arise as described above and is not in practicaluse.

SUMMARY OF THE INVENTION One object of the present invention is toprovide a plasma flame generator in which a desired portion of a plasmaflame can be used suitably by bending the plasma flame in a desireddirection.

Another object of the present invention is to provide a plasma flamegenerator in which the noise AN is made small and S/AN is made large bystabilizing the plasma flame.

The present invention consists of a plasma flame generator in which apair of auxiliary electrodes are disposed so as to encircle a plasmaflame and a DC voltage is applied to said auxiliary electrodes to bendions in the plasma flame.

DESCRIPTION OF THE PREFERRED EMBODIMENT Now, a high frequency torchdischarge generator shown in FIG. 5 is described as an example of thepresent invention. In the first place, a case is described where aplasma flame is stably jetted in the direction perpendicular to thedirection of gravity. Referring to FIG. 5, reference numeral 1 is aplasma flame, 11 a rectangular waveguide, 12 a coaxial waveguide, 13 aninner conductor of the coaxial waveguide, 14 an electrode, 15 an inletfor a sample to be analyzed and plasma forming gas, 16 an insulator, 17a lower correcting electrode, 18 an upper correcting electrode, 19 a DCpower source, 20 a groove, 21 a variable shorting plunger, 22 aninsulating plate which insulates the coaxial waveguide 12 from thecorrecting electrodes 17 and 18 in response to a direct current butcouples the waveguide 12 to the electrodes 17 and 18 by capacitance inresponse to an alternating current, and 24 an insulating washer whichinsulates between a binding screw 23 and the correcting electrodes 17and 18. The coaxial waveguide in which the plasma flame is generated isso placed that its axis becomes perpendicular to the direction ofgravity in order to jet the plasma flame in the direction perpendicularto the direction of gravity.

When an electromagnetic field is produced in the rectangular waveguide11 by supplying microwave power from the lower side of the rectangularwaveguide 11, an electromagnetic field is produced in the coaxialwaveguide by a high frequency current is induced by the electromagneticfield in the inner conductor 13 of the coaxial waveguide 12. Then, thehigh frequency current and a high frequency voltage is induced at thetip of the inner conductor 13, that is at, the electrode 14. Here, thevariable shorting plunger 21 is suitably moved to place it at a positionwhere the high frequency power is effectively consumed in a region wherethe discharge plasma flame l is generated. On the other hand, thedischarge plasma flame 1 is generated by introducing a plasma forminggas through the plasma forming gas inlet into the portion of theelectrode 14. At this time, a sample to be analyzed is introduced intothe discharge plasma flame by dispersing it into the plasma forming gasin the form of gas or spray. The plasma flame 1 thus generated is jettedinto the space between the upper electrode 18 and lower electrode 17which are electrically insulated from each other by the insulator 16 andthe groove 20. When a voltage is applied to these electrodes by the DCpower source 19 in such a polarity that the upper electrode 18 ispositive and the lower electrode 17 is negative, positive ions in theplasma flame drift in the lower direction. Thereupon the positive ionscollide with neutral molecules in the gases, then high temperature gasesin the flame are pushed in the direction of motion of the ions, that is,in the downward direction. When the voltage of the DC power source 19 issuitably set up, the electric force which makes the ions move downwardand a force by the convection of the gases which makes the plasma flamebend upward can be balanced, thereby the plasma flame 1 can bestabilized as if it were jetted upward in the direction of gravity.

Therefore, in a spectroscopic analysis device constructed by combiningsuch a device for bending the flame for excitation source and aspectrometer and detector, the plasma flame can be observed from thedirection of its axis without causing instability and a greater quantityof light can be obtained compared with a case of lateral observationposition, therefore, the width of the slit of the spectrometer can bemade narrow enough and an analysis of good accuracy and detection limitis possible.

The separate correcting electrodes 17 and 18 are not limited to thoseshown in FIG. 5. That is, as is shown in FIG. 6, a plurality of sets ofcorrecting electrodes (l7, 18; 17, I8; 17'', 18'') may be provided onthe tip of the coaxial waveguide axially of the waveguide through rings(22, 22', 22") consisting of insulating material. Adjacent sets of theelectrodes are clamped through the rings by a plurality of screws 23,23' and 23" to securely join them together. The respective sets of thecorrecting electrodes are supplied with the required voltages fromindividual voltage sources 19, 19' and 19" to thereby bend the producedplasma flame in the desired direction with respect to the axis of thewaveguide.

It has been explained that the lateral observation position can bechanged without moving the plasma flame generator according to thepresent invention with reference to the high frequency torch dischargegenerator shown in FIGS. 7a and 7b as an example.

The construction of this device is the same as the device shown in FIG.5 but the coaxial waveguide is placed upward in the direction of gravityso as to jet the plasma flame upward in the direction of gravity. Thegroove 20 serves also as observation port of light. The plasma flame 1generated as in the above-mentioned case explained with reference toFIG. 5 is jetted in the space between the upper electrode 18 (in FIGS.

7a and 7b the left electrode) and lower electrode (in FIGS. 7a and 7bthe right electrode) which are electrically insulated from each other bythe insulator 16 and the groove 20. When a voltage is applied to theseelectrodes by the DC power source 19 in such a polarity that the upperelectrode 18 is positive and the lower electrode 17 is negative,positive ions in the plasma flame drift in the right direction, that is,to the side of the lower electrode 17. Thereupon, the positive ionscollide with neutral molecules in the gases, then high temperature gasesin the flame are pushed in the direction of the motion of the ions, thatis, in the right direction and the flame is bent in a shape 21 shown inFIG. 7. The degree of the bend of the plasma flame changes with thevariation of voltage applied by the DC power source 19. Since therelative position of the observation window and the plasma flame in thedirection of the radius of the plasma flame is changed by the variationof the degree of the bend of the plasma flame, the lateral observationposition can be changed by changing the voltage without moving thedischarging device such as a coaxial waveguide.

In a spectroscopic analysis device constructed by combining such adevice for bending the flame for excitation source and a spectrometerand detector, the desired lateral observation position can be set up byonly changing the DC voltage without making the flame generating portionmove mechanically. Then, the device for changing the lateral observationposition becomes simple and the optimum lateral observation position canbe found quickly, thus the measurement can be carried out rapidly.Furthermore, if the correspondence between the kind of element or kindof sample compound and the optimum lateral observation position, thatis, the DC voltage is once determined by carrying out such measurements,the analysis of many elements can be carried out rapidly with the bestdetection limit and accuracy. Though, the high frequency torch dischargegenerator has been described above as an example, the same effects asthose described above can also be obtained in the case of DC plasma jetsor combustion flames according to the present invention. Further, if thecorrecting electrodes are divided on both sides in the direction of theflames axis and DC voltages are applied as shown in FIG. 6, a stableplasma flame can be obtained by suitably adjusting each DC electricfield even when the flame is substantially not uniform on both sides inthe direction of its axis.

As has been described here above, the analysis can be carried outrapidly with good accuracy and detection limit according to the presentinvention and its practical effect is large.

We claim:

1. A plasma flame generator comprising:

a coaxial waveguide having an outer conductor portion and an innerconductor portion each concentrically disposed along a first axis;

means for supplying microwave power between said inner and outerconductor portions of said coaxial waveguide;

means for supplying a plasma forming gas between said inner and outerconductor portions so as to cause a plasma flame to be generated at saidinner conductor portion upon the interaction of said microwave powerwith said gas; and

means for bending said plasma flame, comprising at least one means,surrounding said plasma flame, for generating at least one electricfield in a direction transverse to said axis whereby the ions of saidplasma flame will interact with said at least one electric field tocause said plasma flame to be bent in a predetermined direction withrespect to said axis.

2. A plasma generator according to claim 1, wherein said at least onemeans for generating at least one electric field comprises a pair ofelectrodes concentrically disposed about said axis and a DC power sourceconnected to said electrodes.

3. A plasma generator according to claim 2, wherein said pair ofelectrodes are separated from each other by a groove disposed parallelto said axis.

4. A plasma generator according to claim 3, further including aninsulator disposed between said electrodes and coaxial waveguide.

5. A plasma generator according to claim 4, wherein said DC power sourceis variable, so as to vary the intensity of said one electric fieldthereby controlling the degree of bend of said plasma flame with respectto said axis.

6. A plasma generator according to claim 1, wherein said at least onemeans for generating at least one electric field comprises a pluralityof means for generating a plurality of electric fields including aplurality of electrode pairs disposed along said axis and insulated fromeach other and from said coaxial waveguide, said electrode pairs eachbeing provided with a separate DC power source for separatelycontrolling the magnitude of the electric field generated by eachelectrode pair whereby said plasma flame may-be adjusted, even when saiflame is not substantially uniform on both sides of said axis.

7. A plasma generator according to claim 6, wherein each electrode ofeach pair is separated from the other electrode of the pair by a groovedisposed parallel to said axis.

8. A plasma generator according to claim 7, wherein each of saidseparate DC power sources is variable.

9. A plasma generator according to claim 4, wherein said groove extendsalong said axis up to said insulator.

10. A plasma generator according to claim 9, wherein said innerconductor portion extends along said axis whereby its tip, at which saidplasma flame is generated, is disposed at approximately the sameposition along said axis as said insulator.

